Most modern photographic reproduction systems and cameras are based on either the use of silver halide film, or the employment of solid state electronic photosensors for image detection to drive a display, or the use of an electron beam tube whose sensing surface receives an image and is scanned with an electron beam for one-time read-out and separate storage of the detected picture signals. While all of these prior systems work reasonably well, each has certain disadvantages. For example, those cameras using film reguire relatively complex shutter mechanisms, and the film, which is not reusable, must be developed chemically in order to obtain the picture. Electronic solid state sensor-type cameras tend to be relatively large and complicated machines which are relatively expensive to make. Those electronic cameras which utilize an electron beam tube, such as a vidicon tube require a video tape or other storage medium to reproduce the pictures acquired by the tube. That is, they store the picture information in analog or digital form on a separate magnetic medium. That medium imposes significant limitations on the amount of image information that can be stored, thus limiting the quality of the reproductions made from the sensed data. A disc or tape buffer memory also makes that type of camera quite bulky, costly and necessitates large electrical power supplies. It would be desirable, therefore, to provide a new type of camera which can take a series of snapshots electronically and reproduce those pictures without the need to store the picture information on buffer storage media, such as video tapes or video disks.
There have been some efforts in the past to produce materials which can sense and simultaneously store optical images on multiple layer materials and subsequently produce hard copy output. One of these approaches called the Katsuragawa process and its derivative, the so-called Canon NP process, were developed to form electrostatic images for office copying products. Both of these old processes use a photoconductive medium comprising a photoconductive layer and a superimposed dielectric layer. The photoconductive layer modulates an incoming light image to create an electrical charge pattern across the dielectric layer. Toner is then applied to the medium to develop the image. These processes require precise interactions of corona ionic charging of the medium, light exposure while countercharging of the medium and subsequent blanket exposure of the medium to form a stabilized e1ectronic image across the medium's dielectric layer. Furthermore, the Katsuragawa and Canon processes need a gas plasma or open-air environment in order to function; they also require significant amounts of incoming light, i.e., a very intense light image, for exposure because the recording medium used in those processes have relatively poor sensitivities. Also, these prior techniques obtain only limited resolutions and they are not capable of acquiring and storing color images of photographic quality at photographic speeds.
Another approach to the development of an electronic camera system is described in U.S. Pat. No. 3,864,035 (Kuehnle). This camera also utilizes an electrographic recording medium comprising superimposed photoconductive and dielectric layers. The medium is exposed to a light image and simultaneously, a corona-prducing device charges the surface of the medium to a peak voltage dependant upon the intensity of the incoming light. Due to the different light and dark decay characteristics of the medium, an electronic charge distribution develops across the surface of the medium's dielectric layer. That charge pattern corresponds to the incoming light image and immediately toner is applied to the surface of the medium to develop that electrostatic image before it decays. Thus, that patented camera utilizes well known xerographic and electrofax copying techniques and the value of that prior arrangement resides in the packaging of the various components that carry out those well known processes into a small camera-size envelope. As noted, the recording medium described in that patent can store an acquired image for only a very short time so that the image must be developed by toner almost simultaneously with its acquisition. This presents certain timing problems, it also requires that the camera incorporate a complete toning station which increases the size, complexity and cost of the camera.
That prior camera system also has a low photospeed, making it commercially not viable. Large amounts of light are required to create an image on the medium, comparable to the exposures needed in xerographic copiers, e.g., ASA 1.
Another disadvantage of that camera system stems from its utilization of an electrographic recording medium incorporating a substrate or base through which the image being acquired is projected that is made of an organic plastic material, such as polyester, polyethylene or the like. Although that substrate is quite optically clear when the medium is new, due to its low abrasion resistance, its optical properties begin to deteriorate rapidly when the medium is used, because scratch marks accumulate on its surface.
Furthermore, the medium described in that patented system, even if it could retain an acquired image for a reasonable period of time, cannot be read-out by the most effective technique, namely electron beam scanning. This is because such scanning must take place in a vacuum and outgassing from the medium's organic components, particularly the base, produces a sharply reduced vacuum besides causing ion/electron collisions in the scanning beam and effecting the beam electrode, making impossible the retrieval of quality images. Since, out of necessity, development by toner rather than electron beam scanning is utilized in that prior system, it becomes essential that the recording medium is discharged by exposure to a potential near zero in the brightly illuminated areas; only then is it possible to develop the image without pronounced fog in the theoretically clear areas of the picture. This means that the lighter areas of the medium require saturation exposure to accommodate the innate fogging problem with the toner.
Another electronic imaging technique that does permit retrieval by electron beam scanning of an electronically stored image is disclosed in the publication Electrostatic Imaging and Recording by E. C. Hutter et al, Journal of the S.M.P.T.E., Vol. 69, January 1960, pp. 32-35. The recording medium or "phototape" in that reference, also disclosed in U.S. Pat. No. 3,124,456 (Moore), comprises a transparent polyester base coated on one side with a layer of photoconductive material which is, in turn, coated with a thin layer of a dielectric material. To record an image on the medium, the dielectric layer is precharged by a voltage applied across that layer and then the photoconductive layer is exposed to a light image while an electric field is applied across the dielectric layer. The charge on the dielectric layer decays toward zero with the decay being most rapid where the optical image is brightest and, therefore, the photoconductive resistance is the lowest. After a time corresponding to the greatest difference between the potentials in the light and dark areas of the medium, the electric field is turned off and the discharging process stops, thereby 1eaving on the dielectric layer an electrostatic charge distribution corresponding to the optical image incident in the medium. The stored image may be developed by applying toner to the medium or it may be read from the medium by scanning the dielectric layer with a focused electron beam to produce an electrical signal corresponding to the stored image.
Since the Hutter et al system employs a recording medium incorporating an organic plastic substrate, it has the same disadvantages as the patented camera discussed above. Also, in that system, a voltage must be applied to the recording medium prior to exposure in order to precharge the dielectric layer of that medium. Since the precharge bears no relationship to the brightness of the scene, particularly in its dark areas, the image may be totally under- or over-exposed, making it difficult to read. Also, due to imperfections and defects in the medium's active layers, that precharge may vary across the surface area of the medium and is, therefore, not dependable as an exposure reference potential.
That arrangement has several other disadvantages as well which seriously limit, if not prevent, its practical application. More particularly, the phototape used in the Hutter et al system has poor light sensitivity comparable to the very slowest silver halide films, i.e., ASA 1-10. Furthermore, it can store the acquired data on the medium for only a limited period of time, e.g., a few weeks, because of charge leakage in the dielectric layer of the medium. In other words, using a brute force approach, the Hutter et al system achieves exposure of the recording medium along a portion of the charge vs. exposure characteristic curve for that medium yielding only up to eight levels of the grey scale. Accordingly, the quality of the images acquired by that system are not very high. That being the case, it is not surprising that the pictures retrieved from the medium by electron beam scanning are of poor quality and inferior to silver halide film. Furthermore, read-out of the image stored on the medium is accomplished by detecting a capacitively modulated current signal from the medium involving simultaneous movement of many charge carriers in the medium. Resultantly, the resolution of the detected picture signal is less than that of the stored electronic image which, as just stated, was fairly poor to begin with.
Still further, in the process of reading the stored image for display or reproduction, that image is degraded by electronic conduction caused in the medium by the electron beam scanning process itself. In other words, when the Hutter et al system performs a read operation, it also tends to erase the image stored on the medium. This, of course, is completely unacceptable if that system is to be considered for long or short term storage of optical images which may have to be retrieved several times during the storage period.
Other recording systems are disclosed in U.S. Pat. Nos. 3,880,514 and 4,242,433 to Kuehnle et al which do not require precharging of the recording medium. Rather, those systems charge and expose the medium simultaneously. In that respect, they are superior to the Hutter et al recording system; otherwise they are disadvantaged in the same respects as the latter system.